The present invention relates to a method for the damping force adjustment of motor vehicles in dependence on output signals of a transmitter or pick-up arranged at the vehicle body, which are processed and trigger a signal for changing the damping force when exceeding or dropping below a predetermined threshold value.
In a known method of this type (EP No. 151 412-A) the output signals of the transmitter or pick-up constructed as magnitude, respectively, acceleration transmitter or pick-up are subjected to a frequency analysis. A damping force adjustment is then undertaken when the preferred low-frequency component of this output signal which is representative for the vibrations or oscillations of the vehicle body, exceeds a predetermined threshold value.
With the use of an acceleration transmitter or pick-up, the latter is arranged between a bearing plate for a damping- and spring-part and the end of the damping part, whereby the bearing plate in its turn is elastically supported with respect to the vehicle body. With such an arrangement of the acceleration transmitter, the latter receives preferredly the higher frequency component of the occurring vibration or oscillation frequencies typical for the vibrations or oscillations of the vehicle wheel, whereas the low frequency component then possesses a considerably smaller magnitude than the high frequency component. Added thereto is the influence of the output signal of the acceleration transmitter owing to the alternate action between the spring- and the damping-part with the acceleration transmitter itself. The adjustment of the damping force, in its turn, is undertaken in dependence on the portion of the low frequency vibration or oscillation component measured with respect to its amplitude.
In the known method, a reproducible damper adjustment is not possible because, on the one hand, the low frequency vibration or oscillation component is relatively small and can be determined only with difficulty as a result of the arrangement of the acceleration transmitter in the manner described above. By the determination of the amplitude of this component, a more rapid reaction, as is required in particular during the occurrence of individual obstacles, is not readily possible.
The present invention is concerned with the task to provide a method of the aforementioned type which enables in a simple and reproducible manner an indication concerning the occurrence of individual obstacles and thus a rapid change of the spring-, respectively, damping-force of one or several wheels, respectively of one or several axles of the motor vehicle.
The underlying problems are solved according to the present invention in that average values formed from the output signals of a transmitter directly picking up the acceleration of the vehicle body are subtracted from one another and the numerical difference is compared with a threshold value, and in that the average values correspond to a first measuring period which is larger, respectively, to a second measuring period which is smaller than the natural period (reciprocal natural frequency) of the vehicle body.
A rapid reaction especially in case of individual obstacles becomes possible by the consideration of the changes of the output signals with respect to time. With the selection of the two measuring periods in the described form, the technical measurement expenditures can be kept small. Finally, the possibility is realized by taking into consideration the transmitter or pick-up signal in the manner described above to receive without costly frequency analysis only the low frequency component without significant impairment by the high frequency component and to evaluate the same correspondingly.
It is possible to operate with a predetermined threshold value. However, this represents in any case a compromise because significant influencing magnitudes on the output signal of the acceleration transmitter remain without consideration. It involves thereby the influences due to velocity, respectively, load of the vehicle, respectively, damper oil temperature in case of hydraulic dampers. By taking into consideration the load, respectively, the velocity, the fact is taken into account that at higher loads, respectively, higher velocities, a more rapid response is necessary. This is achieved by a corresponding reduction of the threshold value. By taking into consideration the damper oil temperature, it becomes possible to include the differing damping force adjustment caused by the differing temperature-dependent viscosity of the oil used in damping elements. This leads to the fact that at lower oil temperatures, the threshold value can be selected relatively high because, in this case, a harder damping force adjustment exists anyhow.
The determination of the average values can take place in different ways. One possibility consists in averaging the output signals of the transmitter over measuring periods which are larger, respectively, smaller than the natural period (reciprocal natural frequency) of the vehicle body. Another possibility resides in actualizing the already present average value with the assistance of the actual value. For that purpose the actual value is weighted. Different weighting factors are thereby necessary for the two average values. The relationship of the two weighting factors for the average values is reciprocal to the relationship of the determinative measuring periods for the averaging methods indicated above.